1. Field of the Invention
The invention relates to a pacemaker assembly having redundant ventricular inhibited and ventricular triggered pacemaker units.
2. Description of the Prior Art
The use of implanted cardiac pacemakers has received wide acceptance in the medical field in the treatment of patients having certain heart problems to prevent the patient's heart rate from falling below a predetermined number of beats per minute. Some patients are totally dependent on the pacemaker to cause their hearts to beat while others use their pacemakers only when their heart cannot maintain a heart rate that is sufficiently fast to supply an adequate amount of blood to the body.
Chemical body changes, drug levels, and disease can alter the heart's condition after a pacemaker has been implanted in a patient. It has been proven clinically that it is important that a pacemaker not compete with the patient's heart activity, i.e., not fire at random while the heart is adequately beating on its own. Thus a sick heart suffering from a lack of oxygen can be damaged by or caused to go into fibrillation if stimulated during the repolarization or T-wave cycle of the heart beat.
To prevent this, pacemakers having different modes of pacing are in use. Pacemakers, for example, may function in the ventricular inhibited or demand mode. This type of pacemaker is designed to operate at a preset or programmable number of beats per minute. For example, 70 beats per minute corresponds to a time interval between beats of 857 milliseconds. If a heartbeat is not detected within that period of time, the pacemaker fires causing a heartbeat. If a heartbeat is sensed during the preset time interval, the packemaker is automatically reset in response to the sensed heart activity and waits until the preset time interval again lapses before firing, if a heartbeat is not sensed, or resetting if a heartbeat is sensed.
Demand pacemakers have the advantage of conserving battery energy when it essentially remains dormant because of the patient's heart rate being faster than the rate at which the pacemaker is set to operate. However, this type of pacemaker has the disadvantage of its operation being subject to being inhibited by picking up signals other than a heartbeat, such as certain electromagnetic interference which causes it to reset even when a heartbeat is not produced by the patient.
Many sources of electromagnetic interference have been identified which can or will cause inhibition of demand pacemakers, which are generally designed to revert to asynchronous operation in the event of electromagnetic interference. In the asynchronous mode, stimulation is provided by the pacemaker at a predetermined rate without modification by spontaneous cardiac rhythm. Asynchronous operation is generally useful only for complete heart block without interpolated premature ventriculated contractions, and this is considered to be an unreliable design feature.
Another mode of pacing is the ventricular triggered mode, also known as R-wave synchronous or R-wave tracking pacemakers. Pacemakers functioning in this mode operate at a preset or programmable number of beats per minute. A stimulus is emitted into each QRS complex and distorts it. The sensing activity of this pacemaker differs from a ventricular inhibited or demand pacemaker in that it discharges a stimulus of energy into each sensed heartbeat that is faster than the pacemaker's preset rate. By stimulating the sensed R-wave it avoids stimulation of the T-wave. Because it does not revert to a dormant state when the heart is beating, it consumes more battery energy and reduces the life of the pacemaker's battery if the patient does have a faster heart rate than the pacemaker's programmed or set rate. However, one particular advantage of this type of pacemaker is that it is not inhibited by electromagnetic interference and consequently the heart will be paced even in the event electromagnetic interference is present.
The advantage of battery conservation in a demand pacemaker is not applicable to a patient totally dependent upon his pacemaker. The disadvantage of a shortened battery life in an R-wave synchronous pacemaker does not apply in the case of a patient totally dependent on his pacemaker.
The various modes of pacemaker operation and implantation techniques are described in medical publications. Attention is directed to the article entitled "Cardiac Pacing and Pacemakers v. Technical Aspects of Implantation and Equipment" by Furman et al, American Heart Journal, Vol. 94, No. 2, pp. 250-259 (August, 1977).